1. Field of the Invention
The present invention relates generally to a non-aqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, and particularly, to a non-aqueous electrolyte secondary battery using mixture of lithium-manganese composite oxide having spinel structure and lithium-transition metal composite oxide containing nickel and cobalt as positive electrode active material in the positive electrode, which is featuring controlled decrease in capacity and high rate discharge characteristics after charge/discharge cycles.
2. Description of the Related Art
Recently, as one of advanced secondary batteries featuring high power and high energy density, non-aqueous electrolyte secondary batteries of high electromotive force, using a non-aqueous electrolyte solution, which are charged/discharged making use of oxidation and reduction of lithium, and featuring light weight, high capacity, excellent charge/discharge cycle characteristics have come into practical use.
In such non-aqueous electrolyte secondary batteries, lithium-transition metal composite oxides including lithium-cobalt composite oxide such as LiCoO2, lithium-nickel composite oxide such as LiNiO2, and lithium-manganese composite oxide having spinel structure such as LiMn2O4 are generally used as a positive electrode active material in the positive electrode.
Where the lithium-manganese composite oxide having the spinel structure such as LiMn2O4 is used, thermal stability is higher compared with the cases where the lithium-cobalt composite oxide such as LiCoO2 or the lithium-nickel composite oxide such as LiNiO2 is used, however, there have remained problems that the capacity is decreased, hence the non-aqueous electrolyte secondary batteries having high capacity are not attained, and storage characteristics and charge/discharge cycle characteristics are degraded.
Therefore, in recent years, there has been proposed the use of mixture of the lithium-manganese composite oxide having the spinel structure such as LiMn2O4 and the lithium-transition metal composite oxide such as LiNiO2, to increase the capacity of the non-aqueous electrolyte secondary batteries, and to improve the storage characteristics and the charge/discharge cycle characteristics under high temperature conditions (refer to Japanese Patent No. 3024636, Japanese Patent Publication 2002-100358 and Japanese Patent Publication 2002-110253, for instance).
Unfortunately, however, in the non-aqueous electrolyte secondary batteries using the positive electrode prepared by applying the mixture of the lithium-manganese composite oxide having the spinel structure such as LiMn2O4 and the lithium-transition metal composite oxide such as LiNO2 as the positive electrode active material on a positive electrode current collector, a layer of the positive electrode active material becomes hard, and flexibility of the electrode is degraded. Therefore, there have remained problems that the positive electrode active material comes off from the positive electrode current collector, thus the battery capacity is decreased by expansion/shrinkage of the positive electrode active material during charge/discharge.
Especially in recent years, the above-mentioned non-aqueous electrolyte secondary batteries have been used in various ways. Where, for instance, the non-aqueous electrolyte secondary batteries are used in electromobiles, the charge/discharge at high current is repeatedly carried out in short time, thus, in the non-aqueous electrolyte secondary batteries using the positive electrode active material, the battery capacity is remarkably decreased and the charge/discharge cycle characteristics is degraded.
The above-mentioned non-aqueous electrolyte secondary batteries have generally used graphite as negative electrode active material in negative electrodes.
Unfortunately, however, where the charge/discharged is repeatedly carried out to the non-aqueous electrolyte secondary batteries using graphite as the negative electrode active material in addition to the positive electrode active material which is the mixture of the lithium-manganese composite oxide having the spinel structure such as LiMn2O4 and the lithium-transition metal composite oxide such as LiNiO2, particularly where the charge/discharge at high current is repeatedly carried out in short time, there have remained problems that internal resistance in the non-aqueous electrolyte secondary batteries steeply increases and high rate discharge characteristics is remarkably degraded, by deterioration of the graphite as the negative electrode active material and the like.